1. Field of the Invention
This invention relates to a high resistive element constituting an element of integrated circuits and more particularly to a high resistive element constituting a memory cell of static RAMs by being connected to MOS transistors.
2. Description of the Prior Art
As semiconductor integrated-circuit devices have been used in a wide variety of instruments, circuit elements with high reliability that can be readily formed on a semiconductor substrate are required as each element constituting the integrated circuit.
As a means of holding information electrically, there is a static RAM that is composed of MOS transistors capable of reading out and writing information, and this kind of memory devices usually contain load elements with a high resistance in their memory cells.
FIG. 3 shows an electric circuit of a conventional static RAM in which the load element is composed of resistive elements, and FIG. 4 is a fragmentary sectional view of the static RAM shown in FIG. 3. In FIG. 3, reference symbols T1 to T4 are MOS transistors; reference symbols R1 to R4 are load resistors connected in series with the MOS transistors T1 and T2, respectively. The driver transistors T1 and T2 constitutes a flip-flop circuit together with the load resistors R1 and R2. On the other hand, the MOS transistors T3 and T4 are used for reading out of and writing on the flip-flop circuit by means of address signals fed to their gates.
One end of the load resistors R1 and R2 is connected to the drain of the MOS transistors T1 and T2, respectively. The other end of these load resistors is connected to the power source Vcc.
FIG. 4 is a fragmentary sectional view showing the above-mentioned load resistor R1 or R2. In this figure, reference numeral 1 is a silicon substrate, reference numeral 2 is a field oxide film for the separation of neighboring MOS transistors, reference numerals 3 and 4 are source and drain regions of the MOS transistor, on which is formed thermal oxide film 10, reference numeral 5 is a gate made of either a first polysilicon film or a low resistive material such as a metal with a high melting point for constituting the driver transistor T1 or T2, reference numeral 6 is a wiring portion made of a second polysilicon film to which impurities are added at a high concentration, and reference numeral 7 is a load resistor that is prepared by adjusting to a slight trace level the amount of impurities added to a part of the wiring portion 6 made of the second polysilicon film. Moreover, reference numeral 8 is an interlevel insulator formed between the first and second polysilicon films and between these polysilicon films and the aluminum wiring 9.
The load resistors R1 and R2 made of a second polysilicon film that are required to function as circuit elements in a static RAM must have a high resistance of tens of megaohms to hundreds of megaohms or more to reduce the current flowing through the load resistors. Therefore, impurities to be added to the polysilicon films must be limited to a slight trace amount. Moreover, because the load resistor 7 is formed from the second polysilicon film as a continuous body together with the wiring pattern 6, there is a danger that the impurities contained in the wiring portion 6 at a high concentration may diffuse to the high resistive portion 7, which causes a change in the resistance of the load resistor 7. Therefore, to reduce such a change in resistance, the load resistor must have a length of at least 3 to 5 .mu.m, which imposes many restrictions on the miniaturization of semiconductor integrated circuits by the shortening of load resistors.
The conventional load resistor mentioned above is produced by adding a slight trace amount of impurities to the polysilicon films. Therefore, such a load resistor has the following disadvantages; when the integrated circuit is in the operating condition, the resistance of the load resistor varies considerably by about two orders or more of magnitude due to a change in ambient temperature in the range of 0.degree. C. to 100.degree. C. The current flowing through the load resistor, particularly when the integrated circuit is in the stand-by condition, varies approximately in proportion to the variation in the resistance of the load resistor. There is also a restriction on the length of resistors from the viewpoint of an increase in the density of integrated circuits and miniaturization thereof. In addition, there is a secondary problem that the number of manufacturing steps is increased by use of a two-layered polysilicon film, causing a higher production cost.